Spaceflight

It looks like NASA has flown payloads onboard the X-37B before, so I think it is well within possibility that they could get some room on board for Mars samples during a return. They might not even need to "book" a whole flight as long as the mission has room/capacity for the samples aboard, and the sample container could probably hang out in a medium orbit for a while after getting back to Earth, awaiting an X-37B mission to come up at its own leisure since the orbiter could do all of the rendezvous maneuvering on its own